It is known that a series of products which are very attractive to the consumer can be obtained by fermentation of milk with microorganisms. A review of the prior art is given in Bulletin of the International Dairy Federation No. 227/1988. The large number of suitable species of microorganisms offers the possibility of a wide variation in texture and in the taste of the end product; the technique employed also has a great influence on the texture. A common characteristic is that the product contains live microorganisms.
Many of these products contain, optionally in addition to other microorganisms, a combination of Lactobacillus delbrueckii subspecies bulgaricus, and Streptococcus thermophilus. This combination forms the basis for the preparation of yoghurt-like products. Protocooperation occurs in this preparation: if these species are cultured together they stimulate one another's growth. Together they provide the characteristic texture and taste of yoghurt. The customary culture temperature is between 32.degree. and 45.degree. C.; the inoculation percentages vary from 0.025 to 5%.
However, a problem arises in the case of fermentation with said cooperative combination: as the result of the continuing formation of acid and the progressive degradation of protein as a consequence of the presence of the lactobacillus a sour and/or bitter taste can develop when the product is stored. The lactobacillus is, however, indispensable because of its contribution to the flavour and its production of compounds which stimulate the growth of the streptococcus; it is precisely because of the cooperation that a sufficiently rapid formation of acid is achieved which is important for the texture of the product.
Attempts have been made to solve this problem by pasteurizing the product after preparation; however, it is found that an adequate texture can then be maintained only with the aid of added thickeners (Zuivelzicht 38 (1982) 852-854). Of course, the product then also no longer has the characteristic of the presence of live microorganisms in the product ready for consumption.
Attempts have also been made to arrive at a solution to the problem by preparing a product such as yoghurt without the lactobacillus (J. Dairy Research 49 (1982) 147). In order to compensate for the lack of metabolites of the lactobacillus, which results in a poor growth of the streptococcus, growth stimulants such as hydrolysed casein are then added. However, these stimulents differ from the protein fragments which are formed as a result of the action of the lactobacillus; consequently, the contribution of the lactobacillus to the taste is missing and, in respect of the taste, the products can then also be differentiated from those products in which the lactobacillus and the streptococcus have been grown together. Moreover, there is a desire in the foodstuffs industry to avoid as far as possible additives which are obtained by chemical processing.
In European Patent EP 0,148,299 a method is described in which lactobacilli are added, but only after the acidification by the streptococcus in the presence of stimulants, which are not described in more detail, has gone to completion. In this case also there can be no question of protocooperation.
A solution to the problem of continuing acidification and progressive protein degradation due to the presence of the lactobacillus is given in European Patent Application EP-A 0,322,010. According to the method described in this publication, the lactobacillus and the streptococcus are cultured in a fermenter, separated from one another by a semipermeable membrane; free exchange of substances with a relatively low molecular weight takes place between the two compartments. The product which is tapped from the compartment which contains the streptococcus is found to be fully comparable in respect of taste and texture with the fresh product prepared in the traditional way. However, a disadvantage of this method is that special equipment, which is not available in traditional dairy companies, is required to carry it out.